1. Field of the Invention
The present invention relates generally to an apparatus and method for transmitting data in a mobile communication system supporting hybrid automatic retransmission request (HARQ), and in particular, to an apparatus and method for transmitting reverse data in the mobile communication system supporting HARQ.
2. Description of the Related Art
In general, mobile communication systems can be classified into a system supporting only a voice service and a system supporting only a data service. A typical example of such systems includes a code division multiple access (CDMA) mobile communication system. Of the current CDMA systems, an IS-95 system corresponds to the system supporting only a voice service. However, as communication technology has developed and user demand have increased, a mobile communication system has develop into an advanced system supporting a data service. For example, a CDMA2000 system is a typical mobile communication system that has been proposed to support both a voice service and a high-speed data service.
In a mobile communication system, data transmission/reception is generally performed in a radio link, and as a result, data may suffer a loss during its transmission. In a non-real-time service, if such a data loss occurs, the defective data must be retransmitted. That is, in a voice service, which is a typical real-time service, even though a data loss occurs, it is not necessary to retransmit the defective data. However, in a packet data service, which is a non-real-time service, if a data loss occurs, the defective data must be retransmitted to deliver a correct message. Therefore, a communication system capable of data transmission performs data retransmission with one of several automatic retransmission request (ARQ) schemes. The most common ARQ is hybrid automatic retransmission request (HARQ).
Moreover, in a mobile communication system, data transmission can be generally divided into transmission from a base station to a mobile station and transmission from a mobile station to a base station. Commonly, the transmission from a base station to a mobile station is called “forward transmission,” while the transmission from a mobile station to a base station is called “reverse transmission.”
In reverse data transmission, the CDMA2000 mobile communication system obtains reception performance on a specific level by performing power control on a traffic channel. If a reception error occurs during data transmission over the traffic channel, the defective data is retransmitted using a method of retransmitting a radio link protocol (RLP). Such a RLP retransmission scheme is disadvantageous in that a time required from when a reception error occurred to when the defective data is retransmitted is very long, because a receiver cannot process received packet data in a physical layer, and can process the received packet data in a radio link layer higher than the physical layer, or a layer higher than the radio link layer. In addition, in the RLP retransmission scheme, received defective data cannot be reused.
HARQ can resolve the disadvantages of the RLP retransmission scheme. HARQ retransmits a received defective packet in a physical layer. Therefore, HARQ can resolve the long error processing time problem occurring in the RLP retransmission scheme. In addition, because retransmission is performed in the physical layer, the received defective packet data can be reused.
FIG. 1 is a diagram illustrating a structure of reverse channels in a CDMA2000 mobile communication system. As illustrated in FIG. 1, reverse channels for the CDMA2000 system include a reverse pilot channel 101 for transmitting a reverse pilot signal, a fundamental channel (FCH) 103, and a supplemental channel (SCH) 105. The pilot signal transmitted over the reverse pilot channel 101 is a signal that is always transmitted in a reverse direction when a mobile station transmits traffic. A base station controls a reception power level of the pilot signal so that it approaches a target value set by the base station. Traffic data transmitted over the fundamental channel 103 is always set when the supplemental channel 105 is set up, and delivers signaling information to perform reverse outer loop power control. A data rate of the fundamental channel 103 is variable, and each data rate has its own unique traffic-to-pilot power ratio (TPR). For example, as to the TPR, in Radio Configuration 3 of the CDMA2000 system, data rates of 9.6 kbps, 4.8 kbps, 2.7 kbps, and 1.5 kbps are available, and traffic is transmitted at traffic power of 3.75 dB, −0.25 dB, −2.75 dB, and −5.875 dB over pilot power at the data rates.
A TPR value that is changed according to a data rate of the fundamental channel 103 can be replaced with a different value by a factor such as existence/non-existence of the supplemental channel 105 and a data rate of the supplemental channel 105. In addition, because such a TPR value is transmitted using a signaling method, a long time is required in transmitting the TPR value. That is, because variations in a data rate and reception performance are updated by 20 ms, a long time is required in a signaling procedure for changing a TPR value.
The supplemental channel 105 is formed only when there is a service to be transmitted over a supplemental channel. In addition, like the fundamental channel 103, the supplemental channel 105 has a different TPR value at each data rate. First, in a service period of a supplemental channel, the fundamental channel 103 always transmits data, whereas the supplemental channel 105 intermittently transmits data only when data transmission is necessary. Second, the fundamental channel 103 is different from the supplemental channel 105 in data rate. Third, during outer loop power control, the supplemental channel 105 is not considered and only the fundamental channel 103 is considered.
When the fundamental channel 103 and the supplemental channel 105 are provided as stated above, the supplemental channel 105 transmits traffic information, which is user data, while the fundamental channel 103 transmits control information. The control information transmitted over the fundamental channel 103 becomes information for controlling information on traffic transmitted over the supplemental channel 105 and transmission/reception parameters.
FIG. 2 is a signal flow diagram during reverse data transmission in a CDMA2000 mobile communication system supporting HARQ. In addition, FIG. 2 illustrates a signaling flow during initial transmission and retransmission of data over the reverse supplemental channel 105 when the reverse pilot channel 101 and the reverse fundamental channel 103 are set up.
When there is data to be transmitted in a reverse direction, a mobile station (MS) initially transmits the data over a supplemental channel according to a present TPR value in step 201. Then a base station (BS) receives the initially transmitted data, and determines in step 202 whether an error has occurred in the initially transmitted data. If an error has occurred in the initially transmitted data, the base station transmits to the mobile station a NACK signal indicating occurrence of an error in step 203. The mobile station then receives the NACK signal transmitted by the base station in step 204. Upon receiving the NACK signal, the mobile station retransmits the initially-transmitted data at a TPR value negotiated with the base station in step 205. Here, that a TPR value of the supplemental channel 105 is constant means that a ratio of a power level of a reverse pilot signal to a power level of a supplemental channel, both channels being power-controlled, is constant.
If the initially-transmitted data is retransmitted in this way, the base station receives the retransmitted data in step 206. Further, the base station combines the received retransmitted data with the initially-transmitted data and determines whether an error has occurred in the combined data, in step 206.
As described above, if an error has occurred, the base station transmits a NACK signal as it requested retransmission in step 203. However, if it is determined that there is no error in the combined signal, the base station transmits an ACK signal in step 207. Then, in step 208, the mobile station receives the ACK signal and immediately stops retransmission of the packet for which the ACK signal was received. Unlike this, if the number of retransmissions is limited to a predetermined number, the mobile station ends retransmission after performing the retransmission as many times as the predetermined number.
In HARQ described in conjunction with FIG. 2, the same code symbols as those transmitted at initial transmission are transmitted at the same TPR value as that used at the initial transmission. For example, if power of a supplemental channel transmitted at initial transmission is 10 times higher than signal power of a pilot channel, then during retransmission, the same code symbols as code symbols transmitted at initial transmission are transmitted at power 10 times higher than signal power of a pilot channel as done at initial transmission.
Maintaining the TPR value used at the initial transmission even during retransmission is equivalent to not considering a ratio Eb/Nt of energy to interference per bit of the supplemental channel 105 received at initial transmission. Therefore, retransmission power becomes higher or lower than required power. If the retransmission power is higher than the required power, the retransmission power functions as interference to other users, deteriorating a channel environment. In contrast, if the retransmission power is lower than required power, HARQ performance is deteriorated undesirably. In addition, even when a code rate of initial transmission is high, the same code symbols as those transmitted at initial transmission are transmitted during retransmission, so performance improvement through incremental redundancy (IR) becomes impossible.
Herein, “performance improvement by IR” refers to performance improvement achieved by differentiating code symbols transmitted at initial transmission from code symbols transmitted at retransmission when a code rate of initial transmission is high, thereby decreasing the entire code rate during retransmission. That is, “performance improvement by IR” refers to performance improvement by energy gain, and to performance improvement by a variation in the entire code rate, the sum of a code rate of initial transmission and a code rate of retransmission. That is, when the same data as that transmitted at initial transmission is transmitted at the same TPR as used at the initial transmission, during retransmission, performance improvement through IR becomes impossible undesirably. In addition, because the same TPR value is used, power control according to a required TPR value is not achieved during retransmission.
In addition to the method of maintaining a constant TPR even retransmission, there is a possible HARQ method of setting a TPR value during retransmission to a predetermined value. For example, if TPR at initial transmission is 10, TPR at retransmission is automatically set to 5.
Maintaining the same TPR value at both initial transmission and retransmission or using a predetermined TPR value is equivalent to not considering Eb/Nt of a supplemental channel 105 received at initial transmission. As a result, retransmission power becomes higher or lower than required, deteriorating HARQ performance.